This invention relates to electric heating units and more particularly to such units in which coiled resistance heating elements are embedded in insulating bodies.
In U.S. Pat. No. 3,500,444 to W. K. Hesse, et al., a lightweight ceramic fiber insulation is disclosed. Due to energy saving advantages, such ceramic fiber insulation is often used in place of more dense fire brick for furnace linings. Most of these applications are for oil or gas fired furnaces; however, there is increasing interest in using electric heating elements in which heating coils are embedded in situ in ceramic fiber insulating bodies as taught in the Hesse et al. patent. In that patent, a portion of the heating coil is exposed above the surface of the insulating body in order to permit effective transmission and direction of heat.
The requirement that a portion of the coil be exposed beyond the surface of the insulating body leads to great manufacturing difficulties. As taught in Hesse et al. patent, the insulating body is formed by a vacuum activated filtering process in which the coil is positioned against a screen and liquid from a molding slurry is drawn from the slurry past the heating coil through the screen. Ceramic fiber from the slurry is retained by the screen and accumulates around the coil. It is difficult to arrange for each individual convolution of the coil to be partially exposed from the insulating panel.
An object of this invention is to provide a highly efficient heating unit having a heating coil embedded in an insulating body, yet which is not subject to the difficulties of manufacture resulting from exposing each convolution of the coil beyond the surface of the insulating body.
It is among the advantages of this invention that a flattened heating coil may be embedded in the insulating body and be maintained at a substantially lower temperature for a given oven temperature than would otherwise be possible for a totally embedded round coil.